When transmitting information in the form of binary digits or bits, the transmitted bit stream is normally coded before transmission. The received signal is decoded at the receiver, for instance in a viterbi-analyzer and in a manner known per se. The channel over which the information is transmitted is often subjected to interference and the transmission function of the channel is therefore evaluated and estimated in an equalizer. The received signal is processed in the equalizer, which may be a viterbi-analyzer that has been adapted to the channel concerned. The viterbi-analyzer processes the received signal statistically, so that the signal delivered by the analyzer will coincide, as far as possible, with the received signal, in accordance with the maximum likelihood criterion. The bits delivered by the viterbi-analyzer are therefore correct only to a certain degree of probability. A measurement of this probability, a quality factor or so-called soft information, can be calculated and utilized for continued signal processing, thereby enabling signal processing to be improved.
Described in, for instance, U.S. Pat. No. 4,240,156 is a method of calculating such a probability value for each decoded bit. Although signal processing is improved, it is mentioned in the Patent that difficulties are encountered in carrying out the method, difficulties which in part are of a purely mathematical nature. Simplified methods of calculating probability values or factors are also known in which, for instance, a whole sequence of bits is allotted the same quality factor. The signal processing improvements achieved with the simplified methods are less significant than the improvements achieved with those methods in which each bit is allotted a quality factor.